When you setup Lync Online as one of your domain services in Office 365, you need to create few DNS records in your domain's public DNS to configure your domain to use Microsoft Lync Online for instant messaging.
Your DNS records will look like these below (replace "yourdomain" with the domain name you have enabled Lync Online on it)
|Type||Host name||Points to address||TTL|
These records are used by Lync client (desktop and mobile apps) to determine server information for connecting to Lync Online on your Office 365 account.
If for any reason you are not able to create these records in your public DNS, you need to manually configure Lync clients to connect to Lync Online.
To configure Lync 2010 desktop client follow the instructions here
To configure Lync 2010 iPad app go through following steps:
In your iPad navigate to Lync app. enter your email address as Sign-in Address, enter your password and expand More Details.
Turn off the Auto-Detect Server and enter following address in place of Internal Discovery Address and External Discovery Address:
Click on Sign In button.
On the Welcome page click Next.